Printed circuit base plates generally comprise a resinous base having provided thereon a metal foil, such as copper foil, via a heat resistant adhesive layer, and printed circuit boards are obtained by forming a predetermined wiring pattern in the metal foil.
A printed circuit board with a cover layer is obtained by providing a resinous cover layer on the wiring pattern-formed metal foil via a heat resistant adhesive layer.
As the resinous base and cover layer used in a printed circuit base plate or printed circuit board as described above, polyimide films, polyparabanic acid films, extremely thin glass epoxy plates, etc., are popularly used. These heat resistant materials provide circuit boards having high accuracy and high quality. However, they have the defect that they are inferior to polyester film in humidity resistance and water resistance, and are thus somewhat poor in general purpose properties.
Polyester films have recently come into use as a base or cover layer for printed circuits in place of the above heat resistant materials due to their inexpensiveness and comparatively good humidity and water resistance. However, polyester films have the serious defect that they are liable to shrink when heated. Thus, when they are subjected to a high temperature treatment, such as soldering, the base plate or circuit board comprising the polyester film is liable to shrink and form warps and voids between the metal foil and the base, rendering the same unable to be used in forming highly accurate circuit boards. In fact, even where high accuracy is not required, high temperature treatment of a circuit board comprising a polyester is undesirable, and the use of such circuit boards is limited to situations where the polyester is used in only small or narrow portions so that the possible formation of voids causes no practical problem on heating, for example, by using a soldering bit.
Commercially available polyester films are generally manufactured by melting a polyester resin, extrusion molding it by, for example, an inflation method, and mono- or biaxially stretching to increase its mechanical properities such as tensile strength, initial modulus, impact strength, etc. The high heat shrinkage of the polyester film results from the fact that it undergoes molecular orientation in the above manufacturing steps, which is liable to generate internal stress.